Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery petroleum products are produced using the fluid catalytic cracking (FCC) process. An FCC process typically involves the cracking of heavy hydrocarbon feedstocks to lighter products by contacting the feedstock in a cyclic catalyst recirculation cracking process with a circulating fluidizable catalytic cracking catalyst inventory consisting of particles having a mean particle size ranging from about 20 to about 150 μm, preferably from about 50 to about 100 μm.
The catalytic cracking occurs when relatively high molecular weight hydrocarbon feedstocks are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons. Bottoms and coke are also produced.
The cracking catalysts typically are prepared from a number of components, each of which is designed to enhance the overall performance of the catalyst. FCC catalysts are generally composed of zeolite, active matrix, clay and binder with all of the components incorporated into a single particle.
The zeolite component is primarily responsible for converting, e.g., cracking, the hydrocarbon. There are a number of zeolites suitable for this purpose. Suitable large pore zeolites include crystalline alumino-silicate zeolites such as synthetic faujasite, i.e., type Y zeolite, type X zeolite, and Zeolite Beta, as well as heat treated (calcined) derivatives thereof. Frequently used zeolites include ultra stable type Y zeolite (USY) such as that disclosed in U.S. Pat. No. 3,293,192.
Zeolites derived from certain clays are also known. See U.S. Pat. No. 3,459,680. Such zeolites can be prepared in crystalline form by treating clay powder with a silica source under alkaline conditions (referred to as caustic in the '680 patent), with the resulting zeolite combined with matrix precursors to form catalyst suitable for use in the conversion process. Pre-formed clay-containing particulates, such as spray dried microspheres, can also be processed into a size and form suitable for such processes, and the zeolite can be produced in situ within the particulates. See U.S. Pat. Nos. 4,493,902 and 6,656,347. In either process, the resulting zeolite contains metal impurities, e.g., those comprising iron, magnesium, calcium and titanium that are typically present in the clays used to make these zeolites. Such impurities tend to destabilize the silica alumina zeolite structure, thus causing the collapse of the structure. This leads to loss of zeolite surface area, and accordingly leads to loss of catalytic activity in hydrocarbon conversion processes. Furthermore, zeolites made from clay tend to have a smaller crystal size and hence lower hydrothermal stability than zeolites made from synthetic starting materials.